U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Short Research Paper 072, doi:10.3133/of2007-1047.srp072 Elongate summit calderas as Neogene paleostress indicators in Antarctica 1 2 T. S. Paulsen and T. J. Wilson 1Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, WI 54901, USA ([email protected]) 2Byrd Polar Research Center and School of Earth Sciences, The Ohio State University, 108 Scott Hall, 1090 Carmack Road, Columbus, OH 43210 ([email protected]) Abstract The orientations and ages of elongate summit calderas on major polygenetic volcanoes were compiled to document Miocene to Pleistocene Sh (minimum horizontal stress) directions on the western and northern flanks of the West Antarctic rift system. Miocene to Pleistocene summit calderas along the western Ross Sea show relatively consistent ENE long axis trends, which are at a high angle to the Transantarctic Mountain Front and parallel to the N77ºE Sh direction at Cape Roberts. The elongation directions of many Miocene to Pleistocene summit calderas in Marie Byrd Land parallel the alignment of polygenetic volcanoes in which they occur, except several Pleistocene calderas with consistent NNE to NE trends. The overall pattern of elongate calderas in Marie Byrd Land is probably due to a combination of structurally controlled orientations and regional stress fields in which Sh is oriented NNE to NE at a moderate to high angle to the trace of the West Antarctic rift system. Citation: Paulsen, T. S. and T. J. Wilson (2007), Elongate summit calderas as possible Neogene paleostress indicators in Antarctica. in Antarctica: A Keystone in a Changing World – Online Proceedings of the 10th ISAES, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1047, Short Research Paper 072, 6 p.; doi:10.3133/of2007-1047.srp072 Introduction Extensive Cenozoic alkali volcanic provinces occupy the West Antarctic rift system near its western flank, the Transantarctic Mountains, and along its northern flank, the Marie Byrd Land plateau (Fig. 1). Both provinces are comprised of trachytic to basaltic polygenetic shield and stratovolcanoes, and extensive fields of scattered basaltic cinder cones and lava flows (LeMasurier, 1990; Kyle, 1990). Although neotectonic faulting (LeMasurier, 1990; Davey and Brancolini, 1995; Jones, 1997; Salvini et al., 1997) and widespread active or young volcanism characterize the rift and rift flanks, little is known about the Neogene geodynamic evolution of the rift and rift- flank uplift system. The nature of the Neogene to contemporary stress field in Antarctica is largely unknown because seismic events of sufficient magnitude Figure 1. Map of the West Antarctic rift system showing for focal mechanism solutions have not been recorded the location of major volcanic centers (circles) and (Reading, 2006) and there has been little drilling to obtain summit calderas (yellow stars) on polygenetic volcanoes borehole stress data. Other possible indicators of the on the western and northern flanks of the West Antarctic direction of the crustal stress field include the shapes of rift system in Antarctica (modified from LeMasurier, summit calderas on Antarctica's polygenetic volcanoes. It 1990). SVL, south Victoria Land; A, Cape Adare; B, has recently been established that many summit calderas Mount Bursey; C, Coulman Island; CP, Chang Peak; D, on the Earth’s major Quaternary polygenetic volcanoes Daniell Peninsula; E, Mount Erebus; F, Mount Frakes; H, are elongate in a direction parallel to the contemporary Mount Hartigan; HA, Mount Hampton; M, Mount minimum horizontal stress (Sh) direction (Bosworth et al., Moulton; MA, Mount Andrus; MB, Mount Berlin; MBL, 2000; Bosworth et al., 2003; Holohan, et al., 2005). No Marie Byrd Land; MF, Mount Flint; MM, Mount systematic study of the shapes of Antarctica’s major Morning; MO, Mount Overlord; MS, Mount Steere; MT, summit calderas (Fig. 1) has been previously conducted, Mount Takahe; NVL, north Victoria Land; SI, Mount even though many of the summit calderas are clearly Siple; S, Mount Sidley; TM, Toney Mountain. elongate (Fig. 2). This paper presents the first such Elongate summit calderas as paleostress analysis of elongate summit calderas on polygenetic volcanoes along the western and northern margins of the indicators West Antarctic rift system. The goal is to obtain new Summit calderas elongated parallel to Sh tend to be constraints on stress field orientations during Miocene to common in extensional settings (Bosworth et al., 2000; Holocene volcanism, providing insight into the greater Bosworth et al., 2003; Holohan, et al., 2005). Magma issue of the Neogene to contemporary Antarctic intraplate chambers are similar to vacant boreholes that develop stress field. borehole breakouts elongate in the direction of Sh (Bell and Gough, 1979). In the case of calderas, crustal stress 10th International Symposium on Antarctic Earth Sciences fields cause instabilities in the walls of subsurface magma locations, ages, and, wherever possible, the shapes of chambers causing them to spall off and become elongate. summit calderas from LeMasurier and Thomson (1990) Subsequent roof collapse associated with evacuation of and more recent literature (Panter et al., 1994; Capponi et the magma chamber then leads to a caldera elongate in the al., 1997; Harpel et al., 2004). We determined each Sh direction (Bosworth et al., 2000; Bosworth et al., caldera’s long axis orientation, lengths of long and short 2003). Elongate calderas can also develop parallel to Sh axes, and axial ratio (long axis/short axis). We classified because of the influence of the stress field on ring fracture calderas with axial ratios ≥1.1 as elongate and only dip angle (Holohan et al., 2005). It is also possible for compiled information on calderas retaining at least 2/3 of preexisting faults and fractures within the crust to control their original rim. Calderas with entirely inferred shapes magma chamber growth and produce elongate calderas on geologic maps were excluded. The certainty of the such that their elongation does not reflect Sh (Bosworth et shape of each caldera was classified into four quality al., 2000; Bosworth et al., 2003; Holohan, et al., 2005). In rankings, where A>B>C>D (Table 1). A- and B-ranked areas lacking independent stress data, structural control on calderas have definite boundaries on geologic maps and caldera elongation direction can be assessed by are well defined on topographic maps, satellite imagery, comparison to the trends of regional structures. In the or aerial photography. B-ranked calderas are those that absence of structural control on caldera shapes, the have incomplete rims. C-ranked calderas have either elongation direction of summit calderas can be used as an definite or inferred boundaries on geologic maps, with indicator of the Sh direction and temporal constraints on shapes observed on topographic maps, satellite imagery, the age of this direction can be provided if the age of or aerial photography that are likely, but not certain. D- caldera formation is known. ranked calderas have shapes that have been debated by previous authors. The age assigned to each caldera is the age of the youngest pre-caldera lavas, because these lavas mark the approximate timing of the evacuation of the magma chamber and subsequent caldera collapse. Table 1 lists each of the summit calderas, along with their relevant age and dimensional data. Table 1. Location, Shape and Age of Elongate Calderas Results Figure 2. (a) Landsat TM image (25 m resolution) of the Our compilation identified twenty-nine major shield Mount Overlord elongate summit caldera in north volcanoes with summit calderas (Fig. 1). The majority of Victoria Land. (b) Oblique aerial photograph of the highly the calderas (n=23) occur on the Marie Byrd Land elongate summit caldera on Mount Hampton in Marie plateau, with the remaining (n=6) along the Transantarctic Byrd Land (from LeMasurier and Thomson, 1990). Mountains and the western Ross Sea. Overall, summit calderas range from 2 to 11 km in diameter (average Methods diameter = 4.6 km) and from 3 to 90 km2 in area (average 2 To determine whether calderas on Antarctica’s major area = 17 km ). Elongate calderas comprise nineteen of polygenetic volcanoes in the McMurdo Volcanic Group the twenty-nine summit calderas (Table 1) and these (along the western Ross Sea) and the Marie Byrd Land include ten A-ranked calderas, two B-ranked calderas, provinces record Neogene Sh directions, we compiled the four C-ranked calderas, and three D-ranked calderas. Four 2 Paulsen and Wilson: Elongate summit calderas as Neogene paleostress indicators in Antarctica of these are extremely elongate with axial ratios ≥1.4 (e.g., Fig. 2), while the remaining fifteen have an average axial ratio of ~1.2. Discussion Western Ross Sea & Transantarctic Mountains All six summit calderas in the McMurdo Volcanic Group of the western Ross Sea and Transantarctic Mountains are elongate and show relatively consistent ENE long axis trends, with the possible exception of the calderas on Coulman Island and Daniell Peninsula. The long axes of the Pleistocene summit calderas on Mount Morning, Mount Erebus, and Cape Adare are perpendicular to the trend the Transantarctic Mountain Front fault zone, inferred to follow the coastline (e.g., Tessensohn and Worner, 1991). This suggests that the elongate summit caldera shapes are not structurally controlled and may be due to stress-induced magma chamber elongation. The S30ºE to S60ºE elongate summit caldera on Mount Morning is broadly similar to the S62ºE Sh direction indicated by the Pleistocene vent alignments on its northern flank (Paulsen and Wilson, 2003). In north Victoria Land, the N81ºE elongate summit caldera on Mount Overlord is not parallel to the northeast or northwest faults that dominate the area (Salvini et al., 1997), suggesting that the caldera elongation direction records a Miocene Sh direction. If the calderas on Coulman Island and Daniell Peninsula are elongate NNE as suggested by Capponi et al.